Self-propelled pneumatic tools for making small diameter holes through soil are well known. Such tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose which enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental forward movement through the soil. Exhaust passages are provided in the tail assembly of the tool to allow spent compressed air to escape into the atmosphere.
Most impact boring tools of this type have a valveless air distributing mechanism which utilizes a stepped air inlet. The step of the air inlet is in sliding, sealing contact with a tubular cavity in the rear of the striker. The striker has radial passages through the tubular wall surrounding this cavity, and an outer bearing surface of enlarged diameter at the rear end of the striker. This bearing surface engages the inner surface of the tool body.
Air fed into the tool enters the cavity in the striker through the air inlet, creating a constant pressure which urges the striker forward. When the striker has moved forward sufficiently far so that the radial passages clear the front end of the step, compressed air enters the space between the striker and the body ahead of the bearing surface at the rear of the striker. Since the cross-sectional area of the front of the striker is greater than the cross-sectional area of its rear cavity, the net force exerted by the compressed air now urges the striker backwards instead of forwards. This generally happens just after the striker has imparted a blow to the anvil at the front of the tool.
As the striker moves rearwardly, the radial holes pass back over the step and isolate the front chamber of the tool from the compressed air supply. The momentum of the striker carries it rearwardly until the radial holes clear the rear end of the step. At this time the pressure in the front chamber is relieved because the air therein rushes out through the radial holes and passes through exhaust passages at the rear of the tool into the atmosphere. The pressure in the rear cavity of the striker, which defines a constant pressure chamber together with the stepped air inlet, then causes the striker to move forwardly again, and the cycle is repeated.
In some prior tools, the air inlet includes a separate air inlet pipe which is secured to the body by a radial flange having exhaust holes therethrough, and a stepped bushing connected to the air inlet pipe by a flexible hose. These tools have been made reversible by providing a threaded connection between the air inlet sleeve and the surrounding structure which holds the air inlet concentric with the tool body. See, for example, Sudnishnikov et al. U.S. Pat. No. 3,756,328 and Wentworth et al. U.S. Pat. Nos. 5,025,868 and 5,199,151. The threaded connection allows the operator to rotate the air supply hose and thereby displace the stepped air inlet rearwardly relative to the striker. Since the stroke of the striker is determined by the position of the step, i.e., the positions at which the radial holes are uncovered, rearward displacement of the stepped air inlet causes the striker to hit against the tail nut at the rear of the tool instead of the front anvil, driving the tool rearward out of the hole. Sudnishnikov U.S. Pat. No. 3,616,865 describes a screw-reverse tool wherein exhaust is ported through a central tube that extends in parallel with the compressed air inlet.
Screw reverse mechanisms have obvious limitations. Rotating the hose can become difficult if the tool has traveled far underground, and in any case the tool cannot be switched to reverse rapidly. For this reason, several reversing mechanisms have been proposed which use a second source of compressed air in order to actuate a valve in the tool in order to switch to reverse. See Schmidt U.S. Pat. No. 4,250,972, Spektor U.S. Pat. No. 5,226,487 and Wilson U.S. Pat. No. 5,172,771. A tool described in Kostylev U.S. Pat. No. 4,683,960 provides a central port in the middle of the step to exhaust air sooner than normal when the valve is open and divert compressed air through the central port when the valve is closed, but the valve is operated manually by pulling on a cable. A spring biases the valve to the closed position.
A further reversing mechanism described in Spektor U.S. Pat. No. 5,311,950 reverses upon lowering of the pressure of compressed air. The described tool, however, requires many different parts designed to be assembled in a complex manner.
Despite the availability of many alterative reversing mechanisms, a need remains for a system that is simple, easy to use, reliable, and operable by remote control rather than rotating a hose or pulling on a cable. The present invention addresses this need.